1. Field of the Invention
The present invention relates to an electrode wire for use in electrical discharge machining and the method of manufacturing the same, particularly to a porous electrode wire having an improved machining speed and the method of manufacturing the same.
2. Description of the Background Art
FIG. 1 represents a schematic drawing of a wire electrical discharge machine. An electrode wire is inserted through a start hole(7) of a workpiece(1), which is continuously fed through the hole. A high frequency voltage is applied between the wire(2) and the inside of the hole(7) to initiate an arc discharge between them. Then, machining the workpiece(1) to a desired shape can be achieved by melting the workpiece during the arc discharge and by removing the machining particles using a machining liquid and an instantaneous vaporization power between the wire and the workpiece. In accordance with the machining principle, the wire electrical discharge machine includes a power supply(6), a wire transfer means, a workpiece moving means and a circulating means of the machining liquid.
In general, the workpiece moving means, as indicated by the arrow in FIG. 1, moves during the machining of a workpiece on a plane perpendicular to the wire feeding direction. The wire(2) from a supply spool(3) travels to a take-up roll(4) through a wire transfer means including the upper and the lower guide rollers(5 and 5') of the workpiece.
Then, a high frequency voltage is applied between the workpiece(1) and the electrode wire(2) to start the machining of the workpiece. At the same time, a machining liquid of deionized water is supplied to the machining area to remove the heat of the machining. The machining efficiency, in particular the machining speed, significantly depends on machining parameters such as the feeding speed of the machining liquid, machining current, and the shape and frequency of the machining voltage, and it is known to improve the machining efficiency through a control of the machining parameters.
Since copper has a high electrical conductivity and is easy to form fine wire due to its high elongation property, a copper wire was used initially. However, it revealed many deficiencies mainly due to its low mechanical strength. For example, high tensile strength could not be applied to the copper wire during the machining so that vibration of the wire can not be controlled, resulting in an inferior machining accuracy and tendancy of wire breakage. Moreover, machining speed was relatively slow. Therefore, a molybdemum wire or a tungsten wire as a high strength wire has been used for a special application of a high precision machining. A brass wire having 63-67 wt % copper and 33-37 wt % zinc has been developed for the general purpose of wire electrical discharge machining.
The brass wire has a tensile strength about twice to a copper wire and the machining speed can be improved due to the presence of zinc content in the alloy, which provides a stable discharge and a vaporization power during the machining.
Moreover, as the application field of the wire electrical discharge grows up, it was required for the brass wire to further increase the tensile strength and to improve the machining speed. Therefore, elements such as Al and/or Si can be added to a brass wire to improve the tensile strength and machining speed.
An the other hand, it was known that the machining speed of a brass wire increases when zinc content includes more than 40 wt % in the brass. However, in that case, drawing process to form a wire becomes difficult because of the presence of a brittle phase in the alloy.
U.S. Pat. No. 4,287,404 discloses a zinc coated wire on copper or brass core and the method of manufacturing the same. On a core material having relatively high tensile strength or high electrical conductivity such as copper, brass or steel, a coating material having a relatively low vaporization temperature such as zinc, cadmium, tin, antimony, bismuth or the alloy was electroplated to form a coated wire. According to the wire and the method, the core allows to maintain required mechanical strength or conductivity, and the coating increases cooling ability and flushability because of its relatively low vaporization temperature, thereby improving machining speed and accuracy. Further, the coating material vaporizes easily by the heat during the machining, it protects core material because of the cooling effect of the coating material. Thus, the method of manufacturing the coated wire may include the coating step of zinc electroplating after the final sizing the wire or prior to the final sizing of the wire.
A method of improving the performance of a coated wire was disclosed in U.S. Pat. No. 4,977,303. According to the patent, the method includes steps of; on a metallic core, a coating step of zinc, cadmium or the alloy which forms mixed alloy layer with the core after heat treatment by diffusion annealing; a heat treatment step of the coated wire at 700.degree. C. in an oxidizing atmosphere to form a mixed alloy layer between the core material and the coating material, for example copper-zinc alloy and drawing the coated wire accompanying a mechanical hardening. The coated wire by the method includes a core, a mixed alloy layer and an outer oxide layer. At this time, the oxide layer prevents possibility of short circuits between the wire and the workpiece during the electrical discharge machining, which is not directly related to the machining speed. The improvement in the machining speed of the wire is known to lie on the heat treatment step forming copper-zinc alloy layer, but the mechanism was not clearly revealed.
U.S. Pat. No. 4,686,153 discloses a coated wire having a copper clad steel core and a coating layer of zinc alloy formed on the core, and the method of manufacturing the same. The high strength of steel in the core can provide a superior machining accuracy and the clad copper can provide a good conductivity to the coated wire. On this copper clad steel, zinc coating is applied by electroplating or hot dip galvanizing followed by heat treatment to form a copper-zinc alloy layer. Particularly, when the zinc content in the alloy layer is in the range of 40-50 wt %, the improvement of the coated wire in machining speed becomes evident compared with a simple zinc coated copper clad steel. The coated wire according the patent includes a copper clad steel core and a copper-zinc alloy layer. At the same time, the zinc content in the alloy layer ranges 10-50 wt %, preferably 40-50 wt %. The method of manufacturing the same includes steps of; a providing step of a copper clad steel core, a zinc electroplating step on the core, a drawing step of the zinc coated core to form a wire having a desirable diameter and a heat treatment step of the wire to convert the zinc coating layer into a copper zinc alloy layer having zinc content of 10-50 wt %, preferably 40-50 wt % in such a manner that the concentration of the zinc is gradually decreased along the radially inward direction. Alternatively, the drawing step may be applied prior to the heat treatment step and the zinc coating may use hot dip galvanizing.